Analysis of Pollutant Dispersion in High-Rise Buildings Under Wind–Thermal Coupling

Controlling pollutant dispersion in high-rise buildings is crucial for public health. Vertical pollutant diffusion in stairwells occurs under thermal and wind effects. However, most existing studies rely on idealized boundary conditions. To address this, this study uses field-measured wall temperatures and a window wind velocity as boundary conditions for transient CFD simulations. We investigate the vertical diffusion characteristics of buoyant (CH4) and dense (CO2) pollutants under thermal pressure, window velocity, and wind–thermal coupling in a high-rise residential building in Taiyuan. Results show an asymmetric “fast-up, slow-down” diffusion under thermal pressure, a relatively symmetric profile under window velocity, and a hybrid pattern under coupling where the upper region is wind-dominated and the lower region resembles thermal-driven diffusion. Wind–thermal coupling most significantly enhances upward diffusion. Using the arrival time of CH4 at the 28th floor (about 15 m above the source floor) as the benchmark, the diffusion rate under coupling is about 200% faster than under thermal pressure alone, and about 50% faster than under the window-velocity condition alone. Differences in density lead to variations in dispersion, with CH4 exhibiting higher rates, concentrations (2–4 orders greater), and a broader influence range than CO2. This work interprets the synergistic regulatory mechanism between driving forces and pollutant density, providing a theoretical basis for ventilation optimization and pollution control in high-rise buildings.